1. Field of Invention
This invention relates to an automatic space take-up device to maintain security of a fastener, and in particular to an automatic space take-up device useful to maintain tension on tie down bolts of buildings.
2. Brief Description of the Prior Art
Most building codes require the use of tie down bolts to secure a building to its foundation, and to secure upper story wall panels to lower story panels in multiple story buildings. These bolts are usually secured to the wall panels of a building with tie-down brackets, or penetrate the plates of a wall panel, and are tightly secured with conventional nut fasteners.
Unfortunately, structural lumber is not dimensionally stable, and shrinks over a period of time, sometimes as much as 3/8 inch, resulting in a loosening of the tie down bolts. Unless some corrective measures are taken, the resultant loosening of the tie down bolts, permits an intolerable rocking of the wall panels during earthquakes, which concentrates destructive forces at the corners of the wall panels, resulting in structural failure of the wall panels. The tie down fasteners, however, are often in inaccessible locations and routine inspection and maintenance are not possible, or even when the fasteners are accessible, are usually overlooked.
Some prior attempts have been made to provide self-expanding washers to accommodate shrinkage in structural lumber. An example of an attempt is shown in U.S. Pat. No. 5,180,268, in which an expandable washer is provided in two expandable parts with a stepped ramp or cam surface. The parts are biased axially apart by a torsional spring. Successful acceptance of this type of adjustable washer depends on its ease of installation, and its ability to maintain integrity over use, free from dirt, debris and insect invasion which can compromise its functioning. The aforementioned patent discloses a pair of retainer clips which hook about opposite sides of the washer, and which are to be removed after its installation. These retainers, however, are difficult to remove from an installed and tensioned installation. Additionally, the washer lacks insect and debris shielding to insure its integrity throughout its useful life. Additionally, the stepped ramp is not infinitely adjustable, but only provides preset incremental tension adjustment.
Another expandable washer is shown in U.S. Pat. No. 4,433,879. This washer lacks a resilient biasing spring, and requires manual adjustment, a requirement which limits its usefulness because of the inaccessibility of most building tie down installations. This washer, as that of the previously mentioned '268 patent is not infinitely adjustable.
Another application of helical ramps in washers is shown in U.S. Pat. No. 4,708,555 in which helical ramps are used to provide a lock washer which is also provided with a helical spring to bias the washer into a locking position.
Various self-expanding washers or shims have been provided in machine applications, usually as seals or retainers for bearings of rotational shafts. Examples of these can be found in U.S. Pat. Nos. 3,285,568, 2,405,889, 1,966,780, 1,953,354, 1,746,978 and 1,347,687. These washers or shims lack retainers to maintain the washer in its compressed configuration during installation, and also lack shielding necessary to prevent infiltration of dirt and insects.